Old magistrate is concerning First Lady (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(historical type of magistrate or judge)
'concerning first lady' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'first' means one lot of letters go next to another.
'lady' becomes 'eve' (woman's name).
're'+'eve'='REEVE'
'is' acts as a link.
(Other definitions for reeve that I've seen before include "mate of the ruff" , "Thread (rope) through a ring, old official" , "Old-style magistrate" , "Chief magistrate (hist.); pass (rope) through hole" , "Former official - female ruff" .)